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LMXB in Globular Clusters: Optical Properties Sivakoff et al. 2007

LMXB in Globular Clusters: Optical Properties Sivakoff et al. 2007. David Riebel & Justice Bruursema. The Cast of Characters. LMXBs (low-mass X-ray binary) Compact stellar remnant (BH or NS) Low-mass companion generally transferring mass through Roche-lobe overflow

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LMXB in Globular Clusters: Optical Properties Sivakoff et al. 2007

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  1. LMXB in Globular Clusters: Optical PropertiesSivakoff et al. 2007 David Riebel & Justice Bruursema

  2. The Cast of Characters • LMXBs (low-mass X-ray binary) • Compact stellar remnant (BH or NS) • Low-mass companion generally transferring mass through Roche-lobe overflow • Globular Clusters contribute ~.1% of the light, but ~10% of the active LMXBs • Globular clusters in early-type galaxies • Taken from ACSVCS + NGC 4697 observations

  3. The Sample • Includes 10 brightest ACSVCS galaxies along with NGC 4697 • 6,758 GC’s, of which 270 have detectable X-ray emission • Used HST-ACS/WFC for GCs • Used CXO for LMXBs

  4. Sample Groups • Detected Sample: All sources with a positive net luminosity (N=270) • SNR Sample: All sources detected at the ≥3σ level (N=160) • Complete Sample: All sources with L≥3.2*1038 erg/s (N=61)

  5. Luminosity and Mass • GC mass was determined by using z band magnitudes as tracers where ψz = 1.45 Mo/Lo • Findings confirm that LMXBs are found more often in brighter GCs • Median Mz= -8.5 without LMXBs • Median Mz= -9.9 with LMXBs • Possible power-law dependence of probability of GC containing a LMXB on mass

  6. Size and Metallicity • rh correlates with (g-z) where “red” GC’s are ~17% smaller than “blue” GCs • So rh,M is more relevant: • Follows from Jordan et al. (2005) • This takes care of color dependence of half-light radii (confirmed by data)

  7. Color • Findings confirm previous ideas that redder GC’s preferentially host LMXBs • Found to be 3.15 ± 0.54 times more likely (however there is considerable scatter between galaxies and the real relation is more continuous) • Possible exponential dependence of probability of GC having LMXB based on color. • Means it is more dependent on environment within cluster rather than formation history of cluster

  8. Size does matter • First direct evidence that LMXBs are found more often in GCs that have smaller half-light radii • Since rh is not correlated to mass, this means LMXBs are found in denser GCs • Median rh= 2.6pc without LMXBs • Median rh= 2.2-2.3pc with LMXBs • Probability that a GC has a LMXB decreases roughly as a power law. • There is a similar, but slightly flatter dependence on half-mass radius

  9. Relaxation Timescale • Found using half-mass radius • Previously thought that you needed more than 5 relaxation timescales to produce a LMXB (trelax > 2.5 Gyr) • But larger sample in this paper shows ~15% have trelax > 2.5 Gyr and in-fact show an opposite trend: • Median th,relax,cor= 1.0 Gyr without LMXBs • Median th,relax,cor= 1.3-1.5 Gyr with LMXBs

  10. Dynamical Rates • Two dynamical rates that could affect formation of LMXBs: • Stellar crossing rate: S  M1/2r-3/2 • Encounter rate: Γh M3/2r-5/2 • Both of these rates are found to be higher in LMXB containing GCs (this is consistent with LMXB GCs being more massive and smaller)

  11. Conclusions • Multiple parameters were fit individually and in concert • The linear dependence of LMXB formation can be ruled out with 99.89% confidence

  12. Open Questions • Several models have been proposed to explain the influence of metallicity • Ivonova (2006) suggests that metal rich stars’ convective zones increase magnetic braking • Maccarone (2004) suggests weaker winds from metal rich stars will impact LMXB formation • Undercount?

  13. References • Bregman et al. (2006) ApJ, 640, 282 • Ivanova (2006) ApJ, 636, 979 • Jordan et al. (2005) ApJ, 634, 279 • Maccarone et al. (2004) ApJ, 606, 430

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